Certain heavy liquid hydrocarbon streams are produced from natural deposits of bitumen in sand or from natural deposits of heavy conventional oil referred to as "heavy oil" or sometimes as "extra heavy oil". These streams are called "production fluids"; the hydrocarbon portion of the stream may be bitumen or heavy oil but, for convenience the term "heavy oil" will be used hereafter to include both such portions. Heavy oil production streams are viscous and do not flow readily except at elevated temperatures. Streams containing these materials also contain volatiles (e.g. natural gas), water and sand, all of which must be separated from the heavy oil in the stream.